The present invention is described below with respect to a liquid crystal shutter. It will be appreciated, though, that features of the invention may be utilized with shutters formed of materials other than liquid crystal and also may be utilized with devices other than shutters. A shutter, as is used herein, refers to a device for controlling intensity of electromagnetic energy or electromagnetic radiation that is being transmitted through the shutter. In the preferred embodiment described in detail below, such electromagnetic energy is in the form of light and more preferably is in the form of light (i.e., electromagnetic energy) that is in the visible spectrum as well as in the various infrared spectra and ultraviolet spectra, all collectively referred to as light below. Such control may be by way of graduated or analog control or intensity of transmitted light preferably without detrimentally affecting the image characteristics of such light. Such control also may be digital, i.e., on, off, and specific intermediate levels of transmission or intensity, etc.
An exemplary liquid crystal shutter with which the driving circuit of the invention may be utilized is disclosed in U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, and Re. 32,521. An example of such shutter includes a pair of linear (plane) polarizers, one being used as an input polarizer and the other as an output analyzer, and a variable liquid crystal optical retarder between the two polarizers. By changing the electric field applied to liquid crystal in the retarder, the plane of polarization (or relationships of the axes of elliptically polarized light) of the light transmitted through the retarder can be changed; and the intensity of light transmitted through the analyzer will be a function of the polarization direction (characteristics) of the light transmitted through the retarder.
A shutter system which may employ such an exemplary liquid crystal shutter is disclosed in copending, commonly owned U.S. patent application Ser. No. 07/653,661 filed Feb. 8, 1991, for "Eye Protection System For Welding Helmets And The Like". The present invention is useful to provide electrical power and to operate such a shutter system.
The disclosures of such patents and patent application are incorporated in their entireties by this express reference thereto.
One approach to providing for high speed operation, for example, in the microsecond range, as compared to the millisecond range, say on the order of 500 microseconds or faster, and preferably in the range of from several microseconds to several tens of microseconds, for the optical shutters of such patents and patent applications, as well as other similar shutters, is according to surface mode effect, whereby that liquid crystal material located near the center of the liquid crystal cell along the optical transmission direction through the cell is maintained in a preferred alignment during cell operation in a clear state, whereas liquid crystal material nearer the respective surfaces of the cell, i.e., the interface between the liquid crystal material and the respective glass plates, is switched to switch between clear and dark states as a function of the applied electric field, as is described in detail in the above-mentioned patents and patent application. Due to birefringence characteristics of the liquid crystal material, for example, nematic liquid crystal, changes in the thickness of differently aligned liquid crystal layers varies retardation effect on transmitted polarized light. Absent adequate field voltage the liquid crystal near the surface aligns generally parallel or slightly tilted with respect to the surface, and such surface "layer(s)" of liquid crystal tend to align with the field when the field voltage is adequately large. In one embodiment some means, such as a bias voltage or electric field, a functionally equivalent rms driving method, or some other means or mechanism, is used to obtain the preferred alignment of liquid crystal material near the center of the cell. In an example, application of a larger voltage/electric field compared to the exemplary bias voltage/electric field, effects switching of the alignment of the liquid crystal material nearer the surfaces.
As is well known, the transition speed for a liquid crystal cell, whether of the twisted nematic type, dyed cell type or surface mode type, is asymmetrical; in particular, such a liquid crystal cell operates faster to achieve an operational condition, e.g., alignment of liquid crystal structure or directors, when driven to that condition by an electric field (or an increase in the field magnitude), than it operates when relaxing to a deenergized or reduced energization state, e.g., reduction or elimination of the electric field. Therefore, for maximum speed of operation to the dark state for eye protection, for example, it is desirable in a welding lens environment that the liquid crystal lens be operated with maximum power to achieve the darkest eye protection state. Also, a surface mode liquid crystal cell usually responds to energization significantly faster than twisted nematic liquid crystal cell, and it, therefore, provides for faster operation in accordance with the present invention.
The exemplary shutter may be used in a variety of embodiments and applications. One example is as a lens or shutter for a welding helmet. Therefore, the terms "lens" and "welding lens" are used synonymously with "shutter" and, as used herein means the device through which an image is viewed without necessarily having any focusing or optical refraction characteristics. The lens or shutter is adjustable to control light, i.e., to increase or to decrease the amount of the incident light which is transmitted through the shutter. When welding is not occurring, the shutter may be substantially optically clear to transmissive or at least minimizes its attenuation of light. When welding is occurring, the shutter may be dark or closed to minimize the amount of light transmitted therethrough in order to protect the eyes of the person performing the welding. In both cases, though, the image characteristics of the light remain intact. A photosensitive device may be used to sense the intensity of light impinging in the area of the shutter so as to provide an input to a drive circuit for the shutter in order to control opening and closing thereof. As is described in the above patent application, a third state or condition of the welding lens or shutter may exist, namely a deenergized state or condition. Such third state preferably is darker than the clear state.
The invention is especially useful for eye protection wherein high speed protective shuttering and protective fail state are desired. Exemplary uses are in welding helmets, spectacles, goggles, and the like, as well as safety goggles for nuclear flash protection, for protection from hazards experienced by electric utility workers and for workers at furnace and electric plant areas and at other places where bright light that could present a risk of injury may occur.
Shade number or shade is the characterization of darkness of a welding lens, for example, (hereinafter sometimes simply referred to as lens); a larger shade number represents a darker, more light blocking (or absorbing) or less optically transmissive lens and a smaller shade number represents a less dark, less light blocking (or absorbing) or more optically transmissive lens. Generally optical transmission means transmission of light and the image or view carried by the light without substantial distortion of the image, e.g., due to scattering. Shade number is a term of art often used in the field of welding and especially welding lenses for eye protection.
Clear state or clear shade means the state of highest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the lowest shade number for the lens.
Dark state or dark shade is the lowest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the highest specified shade number for the lens. The invention is described below in some instances indicating that in the dark state no light is transmitted. While this may be desirable for some applications of the principles of the invention, it will be appreciated that for a welding lens in the dark state there will be some transmission so that the welder can see to do the welding while some light is blocked to provide the desired eye protection from damage, injury or the like by the light emitted during welding.
Shutter response time is the time required for the circuitry associated with the lens to detect a sharp increase in incident light (e.g., due to striking of the welding arc, etc.) and to switch the lens from the clear state to the dark state.
Shutter recovery time is the time required for the circuitry associated with the lens to detect a sharp decrease in light (e.g., due to extinguishing of the welding arc, etc.) and to switch the lens from the dark state to the clear state.
Variable transmittance is the ability of the lens to be switched from one level of luminous transmittance (also referred to as transmission of light) to another level of luminous transmittance in response to a change in incident illumination.
Dynamic operational range or dynamic optical range of the welding lens or shutter is the operational range of the lens between the dark state and the clear state, e.g., the difference between the shade numbers of the dark state and the clear state.
Prior photosensitive devices and circuits for use in automated welding lens systems have not had sensor operational range to function well in both indoor environments and outdoor environments, and they were not automatically adjusting to the relatively gradual changes in ambient light compared to rapid change due to initiation of a welding arc, flame, etc. For example, the difference in light intensity between indoor ambient light and welding light is larger than the difference in light intensity between bright sunlight and welding light, and prior sensor devices and circuits were not able automatically and conveniently to adjust for such different ambient conditions. Prior sensor devices and circuits also did not adjust automatically to accommodate the change in ambient conditions when the door to a room is opened to allow bright sunlight to enter the room and possibly falsely to trigger a detection of welding or to impede proper sensitivity to welding.
It is desirable to minimize the time required for a drive circuit to energize variable liquid crystal shutters to a particular state, e.g., the dark state to expedite protection for a welder's eyes. Using such shutters in a welding helmet and in other environments where there is not a convenient access to a power supply connection directly to a utility company, battery power ordinarily must be used to drive the shutter. It is desirable to minimize the power drain on such battery or other portable power supply for such lenses and other devices to maximize safe long term operation without having to change a battery. It also is desirable to provide a wide dynamic sensor operational range for the photosensitive detector and associated circuitry for an automatic shutter.